The Toa Baja Well was drilled on the coastal
plains of northern Puerto Rico with a total depth of 2705m
[Larue, 1990]. Interstratified limestone, quartz-bearing
calcareous sandstones, and shales dominate the uppermost 580
m and are separated from underlying rocks by an
unconformity. Below this unconformity continuing tototal
depth, lithologies encountered consist of volcaniclastic
sandstones/siltstones, pelagic carbonates, volcanic flows and
either plutonic rocks or coarse-grained immature sandstones
derived from plutonic bodies....

Feeding strategies of earthworms and their influence on soil processes are often inferred from
morphological, behavioral and physiological traits. We used 13C and 15N natural abundance in earthworms,
soils and plants to explore patterns of resource utilization by different species of earthworms in three
tropical ecosystems in Puerto Rico. In a high altitude dwarf forest, native earthworms Trigaster longissimus
and Estherella sp. showed less 15N enrichment (delta 15N = 3–6%) than exotic Pontoscolex corethrurus (15N =
7–9%) indicating different food sources or stronger isotopic discrimination by the latter. Conversely, in a
lower altitude tabonuco forest, Estherella sp. and P. corethrurus overlapped completely in 15N enrichment
(delta15N = 6–9%), suggesting the potential for interspecific competition for N resources. A tabonuco forest
converted to pasture contained only P. corethrurus which were less enriched in 15N than those in the forest
sites, but more highly enriched in 13C suggesting assimilation of C from the predominant C4 grass. These
results support the utility of stable isotopes to delineate resource partitioning and potential competitive
interactions among earthworm species. Copyright # 1999 John Wiley & Sons, Ltd.

Dual stable isotope analyses (delta 13C and delta 15N) of fiddler crabs from a forest-fringed, land-locked lagoon in Puerto Rico indicated the differential assim- ilation of material from ingested sediments. Fiddler crabs preferentially selected foi niitrogen-fixing benthic microalgae (cyanobacteria) over vascular plant detritus. These results question the assumption that mangrove detritus is always the prin- cipal source of energy to estuariiie consumers. Previous research fiom this lagoon as well as from Amazonia suggests that the magnitude of lan-d-wvater ecotonal coupling may be low for these particular tropical systems where benthic algal productivity is presumably high.

Many studies of climate variability in the
Tropical Ocean have used high-resolution chemical
tracer records contained in coral skeletons. The complex
architecture of coral skeletons may lead to the
possibility of biases in coral records and it is therefore
important to access the fidelity of coral geochemical
records as environmental proxies. Coral skeletal records
from the same coral colony, and even the same
corallite, may show large variation due to differing
extension rates, formational timing of the skeletal elements,
colony topography, and sampling resolution. To
assess the robustness of the skeletal record, we present
d13C and d18O data based on different sampling resolutions,
skeletal elements, and coral colonies of Montastraea
faveolata species complex, the primary coral
used for climate reconstruction in the Atlantic. We
show that various skeletal elements produce different
isotopic records. The best sampling rate to resolve the
full annual range of sea surface temperature (SST) is 40
samples per year. This sampling frequency also consistently
recovered SST variability measured at weekly
intervals. A sampling rate of 12 times per year recovered
84% of the annual range recording average
monthly SST changes through the year. Six samples per
year significantly decreased the ability to resolve the
annual SST range. The d18O recorded from two adjacent
colonies were very similar, suggesting that this
isotope can be trusted to record environmental changes.
The d13C, on the other hand, remained highly variable,
perhaps as a result of the activity of symbiotic algae
(zooxanthellae).

We examined natural abundances of "3C in vegetation and soil organic matter (SOM) of subtropical wet and rain forests to characterize the isotopic enrichment through decomposition that has been reported for temperate forests. Soil cores and vegetative samples from the decomposition continuum (leaves, new litter, old litter, wood, and roots) were taken from each of four forest types in the Luquillo Experimental Forest, Puerto Rico. SOM 613C was enriched 1.6%o relative to aboveground litter. We found no further enrichment within the soil profile. The carbon isotope ratios of vegetation varied among forests, ranging from -28.2%o in the Colorado forest to -26.9%o in the Palm forest. Isotope ratios of SOM differed between forests primarily in the top 20 cm where the Colorado forest was again most negative at -28.0%o, and the Palm forest was most positive at -26.5%o. The isotopic differences between forests are likely attributable to differences in light regimes due to canopy density variation, soil moisture regimes, and/or recycling of CO2. Our data suggest that recalcitrant SOM is not derived directly from plant lignin since plant lignin is even more "3C depleted than the bulk vegetation. We hypothesize that the anthropogenic isotopic depletion of atmospheric CO2 (ca 1.5%o in the last 150 years) accounts for some of the enrichment observed in the SOM relative to the more modern vegetation in this study and others. This study also supports other observations that under wet or anaerobic soil environments there is no isotopic enrichment during decomposition or with depth in the active profile.

Conversion of abandoned cattle pastures to secondary forests and plantations
in the tropics has been proposed as a means to increase rates of carbon (C) sequestration
from the atmosphere and enhance local biodiversity. We used a long-term tropical reforestation
project (55–61 yr) to estimate rates of above- and belowground C sequestration
and to investigate the impact of planted species on overall plant community structure.
Thirteen tree species (nine native and four nonnative species) were planted as part of the
reforestation effort in the mid to late 1930s. In 1992, there were 75 tree species (.9.1 cm
dbh) in the forest. Overall, planted species accounted for 40% of the importance value of
the forest; planted nonnative species contributed only 5% of the importance value. In the
reforested ecosystem, the total soil C pool (0–60 cm depth) was larger than the aboveground
C pool, and there was more soil C in the forest (102 6 10 Mg/ha [mean 6 1 SE]) than in
an adjacent pasture of similar age (69 6 16 Mg/ha). Forest soil C (C3-C) increased at a
rate of ;0.9 Mg·ha21·yr21, but residual pasture C (C4-C) was lost at a rate of 0.4 Mg·ha21·yr21,
yielding a net gain of 33 Mg/ha as a result of 61 years of forest regrowth. Aboveground
C accumulated at a rate of 1.4 6 0.05 Mg·ha21·yr21, to a total of 80 6 3 Mg/ha. A survey
of 426 merchantable trees in 1959 and 1992 showed that they grew faster in the second
33 years of forest development than in the first 22 years, indicating that later stages of
forest development can play an important role in C sequestration. Few indices of C cycling
were correlated with plant community composition or structure. Our results indicate that
significant soil C can accumulate with reforestation and that there are strong legacies of
pasture use and reforestation in plant community structure and rates of plant C sequestration.

Tropical small mountainous rivers (SMRs) may transport up to 33% of the total
carbon (C) delivered to the oceans. However, these fluxes are poorly quantified and
historical records of land-ocean carbon delivery are rare. Corals have the potential to
provide such records in the tropics because they are long-lived, draw on dissolved
inorganic carbon (DIC) for calcification, and isotopic variations within their skeletons are
useful proxies of palaeoceanographic variability. The ability to quantify riverine C inputs
to the coastal ocean and understand how they have changed through time is critical to
understanding global carbon budgets in the context of modern climate change. A seasonal
dual isotope (13C & 14C) characterization of the three major C pools in two SMRs and
their adjacent coastal waters within Puerto Rico was conducted in order to understand the
isotope signature of DIC being delivered to the coastal oceans. Additionally a 56-year
record of paired coral skeletal C isotopes (δ13C & Δ14C) and trace elements (Ba/Ca,
Mn/Ca, Y/Ca) is presented from a coral growing ~1 km from the mouth of an SMR. Four
major findings were observed: 1) Riverine DIC was more depleted in δ13C and Δ14C than
seawater DIC, 2) the correlation of δ13C and Δ14C was the same in both coral skeleton
and the DIC of the river and coastal waters, 3) Coral δ13C and Ba/Ca were annually
coherent with river discharge, and 4) increases in coral Ba/Ca were synchronous with the
iii
timing of depletions of both δ13C and Δ14C in the coral skeleton and increases in river
discharge. This study represents a first-order comprehensive C isotope analysis of major
C pools being transported to the coastal ocean via tropical SMRs. The strong coherence
between river discharge and coral δ13C and Ba/Ca, and the concurrent timing of increases
in Ba/Ca with decreases in δ13C and Δ14C suggest that river discharge is simultaneously
recorded by multiple geochemical records. Based on these findings, the development of
coral-based proxies for the history of land-ocean carbon flux would be invaluable to
understanding the role of tropical land-ocean carbon fluxes in the context of global
climate change.

Tropical forests are an important source of atmospheric methane (CH4), and recent work suggests that CH4 fluxes from humid tropical environments are driven by variations in CH4 production, rather than by bacterial CH4 oxidation. Competition for acetate between methanogenic archaea and Fe(III)-reducing bacteria is one of the principal controls on CH4 flux in many Fe-rich anoxic environments. Upland humid tropical forests are also abundant in Fe and are characterized by high organic matter inputs, steep soil oxygen (02) gradients, and fluctuating redox conditions, yielding concomitant methanogenesis and bacterial Fe(III) reduction. However, whether Fe(III)-reducing bacteria coexist with methanogens or competitively suppress methanogenic acetate use in wet tropical soils is uncertain. To address this question, we conducted a process-based laboratory experiment to determine if competition for acetate between methanogens and Fe(III)-reducing bacteria influenced CH4 production and C isotope composition in humid tropical forest soils. We collected soils from a poor to moderately drained upland rain forest and incubated them with combinations of C-13-bicarbonate, C-13-methyl labeled acetate ((CH3COO-)-C-13), poorly crystalline Fe(III), or fluoroacetate. CH4 production showed a greater proportional increase than Fe2+ production after competition for acetate was alleviated, suggesting that Fe(III)-reducing bacteria were suppressing methanogenesis. Methanogenesis increased by approximately 67 times while Fe2+ production only doubled after the addition of (CH3COO-)-C-13. Large increases in both CH4 and Fe2+ production also indicate that the two process were acetate limited, suggesting that acetate may be a key substrate for anoxic carbon (C) metabolism in humid tropical forest soils. C isotope analysis suggests that competition for acetate was not the only factor driving CH4 production, as C-13 partitioning did not vary significantly between (CH3COO-)-C-13 and (CH3COO-)-C-13 + Fe(III) treatments. This suggests that dissimilatory Fe(III)-reduction suppressed both hydrogenotrophic and aceticlastic methanogenesis. These findings have implications for understanding the CH4 biogeochemistry of highly weathered wet tropical soils, where CH4 efflux is driven largely by CH4 production.